Step climbs 737NG

[Driver 170]

Reading through the FCTM about step climbs. When can i pilot best judge when to step climb after buring fuel during cruise? Lets say the FMS shows CRZ ALT FL390 OPT399 MAX410. Or any other kind of split it could show between OPT and MAX.

Can you step climb above OPT?

[Chesty Morgan]

Of course you can. There's nothing to stop you step climbing up to max.

[Driver 170]

Done a flight from EGGP to LSGG and my FMS was showing OPT 402 / MAX 410. My OFP planned a cuising FL410. I'm not entirely sure about the risks operating at MAX?

[Chesty Morgan]

With an OPT of 402 your MAX (as in 1.3g margin) would have been above 410 but it'll show 410 as that is the airframe limit.

Off the top of my head the normal difference between OPT and MAX is 1700'. For example, with an OPT of 393 you'd see a MAX of 410 but below that the MAX will also reduce.

[wiggy]

What Chesty said.

Your Max is dictated by your buffet boundary ( which is general terms will climb as the aircraft weight reduces) but will be capped at the certified airframe limit.

I'm not entirely sure about the risks operating at MAX?

If you're operating at the weight MAX, not the capped MAX (e.g the FMC shows a MAX of FL400 and that's where you are) then you're at the merge of both high speed bufet and the "pulling 1.3g" low speed buffet. You're not going to fall out of the sky but any manoeuvering needs to be done with caution (BTW some operators use the 1.2 g boundary)..IMHO it's not a comfortable place to be for long.

With the caveat that I'm assuming your FMC is similar to that on slightly earlier Boeings your OPT is, in simple terms, the optimum level at the current weight, but it doesn't take forecast route winds/temps into account. The best level of all might be the RECMD level (if that's displayed on your FMC)...you can certainly climb above both OPT (and RECMD if you have it), and sometimes it's worth doing so for tactical reasons ( most especially on Long sectors).

When can i pilot best judge when to step climb after buring fuel during cruise?

Simplistic approach is to stick the available "step" size in the appropriate place in the FMC CRZ page (e.g. 1000/2000/I/R) and step up when the box tells you to do so.

[Otto Throttle]

In the software load on our FMC, the difference between OPT and MAX is usually around 700ft.

Generally, when planning a step climb, we normally enter a level approx 300ft higher than the planned step level (ie, if we want to step up to FL380, we enter 383 as the desired step FL). It means you wait a little longer for the step climb point, but it also provides a slightly better margin than climbing immediately to MAX level.

[Driver 170]

The reason i chose FL410 was my OFP used this ODD Flight level because in France RVSM airspace flights heading south will use ODD FLs and north EVEN FLs unlike standard ICAO east / west rule.

But the SOPs i follow that i got from a 737NG pilot never allows flying above OPT unless you have a good reason.

Yeh MAX altitude is where an aircraft can achieve the required buffet margin with rate of climb or is thrust limited? Thats what i only know lol

No RCMD level on my FMC just OPT. So always try to operate below MAX ALT by 1000 in rough air and 800 in smooth air so i read?

Would of it been better if i flew below OPT at FL390 and took a slightly penalty instead of FL410?